1. Field of the Invention
The present invention relates to an imaging lens having a fixed focus which is suitable for use in a small-size imaging apparatus, for example, a digital camera using an imaging device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) or the like, and a camera using a silver halide film.
2. Description of Related Art
Recently, as personal computers have become popular in regular families, there has been rapidly spread a digital still camera (which is hereinafter referred to as a digital camera simply) which can input image information about scenes, persons and the like photographed into the personal computer. Also, as the performance of a cellular phone has been enhanced, the incorporation of a module camera for inputting images into a cellular phone (a module camera for a cellular phone) has been increasing.
In these imaging apparatus, there are used imaging devices such as a CCD or a CMOS. In such imaging apparatus, recently, because the imaging device has been downsized, the whole of the imaging apparatus has been also downsized greatly. Also, since the number of pixels included in the imaging device has also been increasing, the resolution and performance of the imaging device have been enhanced.
As an imaging lens which is used in such downsized imaging apparatus, there are known imaging lenses which are disclosed in, for example, the below-mentioned patent reference. In JP-A-10-48516 and JP-A-2002-221659, there is disclosed an imaging lens having a three-lens configuration. In JP-A-2004-302057, JP-A-2005-24581, JP-A-2005-4027 and JP-A-2005-4028, there is disclosed an imaging lens having a four-lens configuration. In the imaging lens disclosed in JP-A-2004-302057, there is interposed an aperture diaphragm between the second and third lenses in order from the object side, and, in the imaging lens disclosed in JP-A-2005-24581, an aperture diaphragm is disposed nearest to the object side.
As described above, recently, as the imaging device has been reduced in size and has been increased in the number of pixels included therein, especially, the imaging lens for a digital camera has been required that it has an enhanced resolution performance and provides a downsized structure. On the other hand, with respect to the imaging lens for a module camera for a cellular phone, there have been mainly required cost performance and compactness; however, recently, in the module camera for a cellular phone as well, the number of pixels included in the imaging device has been increasing and thus the need for the enhanced performance of the module camera for a cellular phone has been increasing.
Thus, there has been desired the development of various lenses which are improved comprehensively in the cost, image forming performance and compactness thereof. For example, there has been desired the development of a low-cost and high-performance imaging lens which not only can secure compactness capable of use in a module camera for a cellular phone but also can provide a high performance capable of use in a digital camera.
To meet the above demands, for example, there can be employed a structure in which, for the purpose of downsizing and cost reduction, there are used three or four lenses and, for the purpose of performance enhancement, an aspherical surface is used positively in the lenses. In this case, the aspherical surface contributes to the downsizing and enhanced performance, but it provides a disadvantage in manufacture and thus their manufacturing costs are easy to increase. Therefore, in use of the aspherical surface, preferably, the manufacturing cost performance thereof may be taken into consideration sufficiently. As regards the lenses disclosed in the above respective references, the three or four-lens configuration is used and aspherical surfaces are used. However, in this lens configuration, for example, the image forming performance and compactness do not go together sufficiently.
On the other hand, in an imaging device for photographing a static image, to reduce signal noise, provision of a shutter mechanism is required. To reduce the shading, preferably, the shutter mechanism may be disposed near to an optical aperture diaphragm; and, to secure its telecentric performance, the optical aperture diaphragm may be preferably disposed nearest to the object side. However, from the before-mentioned viewpoint, when the aperture diaphragm and shutter mechanism are disposed nearer to the object side than a first lens, the whole length of the lenses is large, which provides a disadvantage in downsizing the imaging device. To solve this, there is employed a structure in which the aperture diaphragm and shutter mechanism are disposed in the interior of the lens system, for example, they are interposed between the first and second lenses; however, in this arrangement, it is necessary to secure a sufficient space for arranging the aperture diaphragm and shutter mechanism within the lens system.